(1) Field of the Invention
This invention relates to novel maleimide compounds and to a process for the preparation thereof.
(2) Description of the Prior Art
A variety of heat-resistant resins have been developed as insulating materials meeting the constant demands for greater capacity, miniturization and weight-saving, improved reliability and thermal stability, longer lifetime, maintenance-free properties, etc. of electronic devices and apparatus. Particularly, a resin having an imide group exhibits excellent properties in electrical insulating properties, heat resistance, and dimensional stability for wide industrial uses. For example, polyamide resins and polyamine-modified polymaleimide resins are known in the art as a heat resistant thermosetting resin for use in electrical insulating material. These resins are generally satisfactory in heat resistance, but have such disadvantages that the resins are impractical to be used for such a purpose as a casting resin in that the resins have a low curing speed, that they are brittle due to too high crosslinking density, that they are liable to develop cracks on heating and cooling, and that they have low mechanical strength. Further, maleimides known in the art are only slightly soluble in low-boiling solvents such that these maleimides are required to be dissolved for use in particular high-boiling solvents such as dimethyl-formamide and N-methylpyrrolidone, which results in needing a high temperature for volatilizing the solvents after impregnating a base, in making it difficult to remove the solvent completely from laminates, and in a poor quality of the product with great technical disadvantages.
The process for the preparation of maleimide compounds is known in the art. For example, Japanese Patent Laid-open No. 95960/78 discloses a process for the preparation of bismaleimides, in which bismaleamic acid is reacted with a lower carboxylic anhydride in the presence of a metallic compound and a tertiary amine. The application of the above process to a hydroxyl group containing maleamic acid such as p-hydroxyphenyl maleamic acid results in esterification of the hydroxyl group with the lower carboxylic anhydride without obtaining a maleimide as a product. Therefore, as a process for the preparation of a hydroxyl group containing maleimide compound, such a process that a maleimide compound, hydroxyl group of which has been esterified, is subjected to ester interchange reaction or to hydrolysis has been adopted as in Belgian Pat. No. 613801, for example, which discloses a process for the preparation of N-(4-hydroxyphenyl)maleimide by ester interchange reaction of N-(4-acetoxyphenyl)maleimide. However, the above process takes 14 hours for the ester interchange reaction, and requires a preceeding step for the esterification of hydroxyl group. Further, Japanese Patent Laid-open No. 68770/78 discloses a process for the preparation of a maleimide compound, in which a hydroxyl group containing maleamic acid is subjected to reaction in the presence of an aprotic polar solvent, or of an aprotic polar solvent and an acid catalyst, and water produced as a by-product in the organic solvent is removed out of the reaction system by azeotropic distillation. However, the above process has such disadvantages that solubility of maleamic acid in the organic solvent is low, that a proportion of water to organic solvent in the azeotropic mixture of water and organic solvent is low because water produced as by-product in the formation of maleimide has a high compatibility with the aprotic polar solvent and the acid catalyst so that a large amount of organic solvent is required to obtain the azeotropic mixture, and that it is difficult to obtain maleimide compounds of high quality at a high yield without removing organic solvent completely. The conventional processes described above are those with great industrial disadvantages, and a satisfactory process for the preparation of the hydroxyl group containing maleimide compound has not been obtained yet.